I didn't say to put it all off until the end. The question in the OP was whether to do the programming or the art first, and my advice was to not focus on making art before you know what art you need, in what quantities, and to what specifications. You don't want to spend days meticulously drawing enemy sprites, only to realize later that you really needed them to be 20% smaller. Placeholders are what you use to figure that stuff out.